<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        
    </style>
</head>
<body>
    
</body>
<script>
    
    var a;
    console.log(a)    //undefined
    a = 10;
    console.log(a)    //10
    var b = a;   
    console.log(b);    //10
    var c = a + b;     
    console.log(c);   //20
    var d = c > a;   
    console.log(d);    //true

    a = a + 1    //11
    a += 1;     //12
    a *= 2;    //24
    a /= 3;    //8
    // a %= 3;     //2

    console.log(a);  

    // 计算规则等同于算术运算符



</script>
</html>